The signals recorded with a sensor placed beneath a blood pressure cuff are termed “suprasystolic” signals if the cuff pressure is above the subject's systolic blood pressure. In addition, signals can be recorded when the cuff pressure is below systolic pressure. In all cases, the signals result from pressure energy transmissions and are dependent upon the subject's physiology. FIG. 1 shows typical signals for cuff pressures both above and below systolic, from a piezoelectric sensor.
When the heart pumps, a pressure gradient is generated within the cardiovascular system. This results in pulse pressure waves traveling peripherally from the heart through the arteries. Like any wave, they reflect back off a surface or other change in impedance. Arterial pulse waves reflect back from both the peripheral circulation and from the distal aorta when it becomes less compliant (Murgo, Westerhof et al. 1980; Latham, Westerhof at al. 1985). These reflection waves are identifiable in arterial pressure tracings, but the exact timing and magnitude of the waves are difficult to discern. Nevertheless, they have been the basis of several commercial systems to assess reflectance waves. These systems measure arterial contours using applanation tonometry from the radial artery.
If a low frequency sensor is placed over the brachial artery beneath a blood pressure cuff and the cuff is inflated above systole, suprasystolic signals can be recorded (Blank, West at al. 1988; Hirai, Sasayama et al. 1989; Denby, Mallows et al. 1994). An idealized suprasystolic signal for one heart beat is shown in FIG. 2. These signals contain frequency components of less than 20 Hertz, which are non-audible. Suprasystolic low frequency signals provide clear definition of three distinct waves: an incident wave corresponding to the pulse wave and two subsequent waves. Blank (Blank 1996) proposed that the second wave emanated from the periphery and the relative amplitude of this wave to the incident wave (K1R) was a measure of peripheral vascular resistance (PVR). He proposed a constant such that PVR could be measured from the ratio of the incident to the first reflectance wave. See, also, U.S. Pat. No. 5,913,826, which is incorporated herein by reference in its entirety.
The second suprasystolic wave is, in fact, a reflectance wave from the distal abdominal aorta—most likely originating from the bifurcation of the aorta and not from the peripheral circulation as proposed by Blank. This has been verified in human experiments (Murgo, Westerhof et al. 1980; Latham, Westerhof et al. 1985) and in our studies using pulse wave velocity (PWV) measurements. The relative amplitude of the first reflectance wave is therefore a measure of the stiffness, compliance, or elasticity of the abdominal aorta rather than peripheral resistance.
In the clinical experiments upon which Blank relied to formulate his hypothesis, changes in compliance were induced with epinephrine and epidural anesthesia. The changes in compliance were accompanied by changes in peripheral resistance. Thus, he saw a relationship between his K1R and PVR, but it was a co-variable and not a true association.
The third wave occurs at the beginning of diastole and is believed to be a reflection wave from the peripheral circulation. As such, it is a measure of peripheral vasoconstriction. Suprasystolic signals can be utilized to measure compliance by relating the amplitude of the first wave (incident or SS1) to the amplitude of the second (aortic reflection or SS2) wave. The degree of vasoconstriction can be assessed by measuring the amplitude of the diastolic or third wave (SS3 wave) and relating it to the SS1 wave. Amplitudes, areas under the curves, or other values calculated from the waves can be utilized. Data has been analyzed by measuring amplitudes, ratios of amplitudes and time delays between waves.